In a vehicle mounted with an automatic transmission, an operation of the automatic transmission is controlled in accordance with the shift position selected by manipulation of the driver to a shift position detection device. Specifically, the automatic transmission restricts the range of gear in accordance with the shift position, and changes the gear according to the vehicle speed and the throttle opening degree.
Generally, one shift position is selected by the driver's manipulation of a shift lever, from among a plurality of shift positions including: a parking position (hereinafter, referred to as the “P position”) selected at the time of parking or stopping, a reverse position (hereinafter, referred to as the “R position”) selected at the time of backward running of the vehicle, a neutral position (hereinafter, referred to as the “N position”), and a drive position (hereinafter, referred to as the “D position”) selected at the time of forward running of the vehicle.
For such selection from among a plurality of shift positions, a shift position sensor is disclosed wherein a plurality of position contacts are arranged for detecting the position of the arm that moves along with manipulation of the shift lever (e.g., Japanese Patent Laying-Open No. 11-86680). With this shift position sensor, the shift position is detected based on a contact signal indicating presence/absence of contact of each position contact with the arm.
In the shift position sensor as described above, it is necessary to conduct abnormality detection when there is a failure of a position contact, since detection of the shift position will not be performed correctly on such an occasion. For example, validity of the combination of the contact signals from a plurality of position contacts provided at the shift position sensor may be checked, in which case abnormality can be detected when the combination of the contact signals does not correspond to any of the combinations that can essentially exist.
With such a configuration of abnormality detection, however, abnormality of the shift position sensor cannot be detected as long as the combination of the contact signals when there is a failure in the position contact is among those that can essentially exist, which may result in a delay in detection of abnormality. Further, the current combination of the contact signals needs to be compared with all the possible combinations one by one, leading to an increase in load of abnormality determination.